The present invention relates to a method for identifying the state of an NOx storage catalyst.
The chief emitters of nitrogen oxide (NOx) in the industrial countries are traffic, fossil fuel-fired power stations and industrial plants. Whereas emissions from power stations and from industry are constantly decreasing, the proportion from traffic keeps on becoming more prominent.
NOx emissions from gasoline-driven Otto engines can be drastically reduced by operating at xcex=1 and by cleaning the exhaust gas downstream from the engine using a three-way catalyst. In principle, this possibility does not exist for mixture-regulated Diesel engines which are operated superstoichiometrically. On account of the high oxygen proportion in the exhaust gas, to date no storage catalyst has been realized which can reduce NOx emissions without the addition of reducing agents, such as hydrocarbons or ammonia-forming compounds.
The same is true for lean-operated Otto engines. For that case, there have been vehicles into whose exhaust system branch a storage catalyst is inserted which is operated lean for a certain time (xcex greater than 1) and can store nitrogen oxides during this time. After this xe2x80x9cstorage phase,xe2x80x9d in which the storage catalyst is xe2x80x9cfilledxe2x80x9d with the nitrogen oxides to be stored, as a rule, there follows a much shorter desorption phase, also called xe2x80x9cregeneration phase,xe2x80x9d in which the storage catalyst is xe2x80x9cemptied.xe2x80x9d During the desorption phase, the engine is run in rich operation (xcex less than 1). In reference (1), a storage catalyst is described which is suitable for such requirements. In the literature, the term trap is also used.
In the current concepts for identifying the degree of storage catalyst saturation and the subsequent regulation of fuel/air ratio (air ratio xcex), gas sensors are used which measure the gas to be stored (NOx) downstream from the storage catalyst. Penetration of the gas beyond the storage catalyst indicates in these cases, that the storage catalyst is saturated with the gases to be stored, and that xe2x80x9cemptyingxe2x80x9d (the so-called desorption phase) has to be initiated. Patent documents on NOx sensors (e.g., European Published Patent Application No. 0 257 842, U.S. Pat. No. 5,466,350, German Published Patent Application No. 43 08 767) and other publications (e.g., (2) or (3) as review articles) exist in large numbers. However, such sensors do not detect the degree of storage catalyst saturation, but rather the content of NOx in the exhaust gas. In addition, many of these sensors have stability problems, and besides NOx, they are also cross-sensitive to oxygen and/or water or carbon dioxide. Reference (4) describes a combination sensor which can be built in behind the storage catalyst, which can detect both the air ratio and the NOx content in the exhaust gas, and which is thus suitable for controlling the system of a lean-operated Otto engine. It has to be considered a disadvantage of this method that, only after penetration of the storage catalyst has already occurred can a signal be given to the engine control to xe2x80x9cclear up.xe2x80x9d In addition, the sensor is very expensive. Such a sensor does not show directly the storage state (filling status, degree of filling).
As described in German Published Patent Application No. 199 54 549, however, an operating strategy can be set up which presupposes the presence of a loading sensor, thus achieving more accurate regulation.
German Published Patent Application No. 198 05 928 describes a direct method for identifying the degree of storage catalyst saturation, which identifies the change in the storage catalyst coating (hereafter also designated as storage medium because of its storage properties) on account of a chemical interaction of the nitrogen oxides to be stored with the coating, and which thereby detects the filling state of the storage catalyst.
As described in German Published Patent Application No. 198 05 928, the chemical state of the storage catalyst coating of the monolith, typically made of washcoat, noble metals and storing elements, changes as a function of the degree of saturation, because of the chemical interactions which appear during xe2x80x9cstoragexe2x80x9d and xe2x80x9cemptying.xe2x80x9d With these changes of the coating the physical properties change too, as, for example, the dielectric constant, the electrical conductivity, the refractive index, etc., which can be detected as described in German Published Patent Application No. 198 05 928. German Published Patent Application No. 198 05 928 also describes placing a substitute material having identical or similar properties as the monolith on a suitable transformer, and operating it as storage catalyst state identification sensor.
As described in German Published Patent Application No. 198 05 928, detection of the degree of saturation can be achieved by determination of the complex electrical impedance Z, which also includes the electrical direct current resistance. By complex electrical impedance Z. German Published Patent Application No. 198 05 928 describes the sum of the real part Re(Z) and the imaginary part Im(Z) of the complex impedance Z. The complex electrical impedance Z changes with the measuring frequency applied. The range between 0 Hz (d.c. voltage) and an upper limiting frequency at which the wavelength corresponding to the measuring frequency is substantially smaller than the dimensions of the measuring arrangement, is suitable as the frequency range. As described in German Published Patent Application No. 198 05 928, preferably a suitable measuring frequency is selected, and at this frequency, the complex impedance of the real part and the imaginary part is determined, or a measuring signal is recorded, derived from both or one of these two quantities.
German Published Patent Application No. 199 16 677 describes measuring the dielectric constant of the monolith, which, according to German Published Patent Application No. 199 16 677, differs from ∈r=8.53 in the unloaded state (barium carbonate is present, according to German Published Patent Application No. 199 16 677) to ∈r=4.95 in the loaded state (barium nitrate).
German Published Patent Application No. 199 16 677 and German Published Patent Application No. 198 05 928 have in common that both use a signal which indicates the loading state. It is true that with such a measuring device one can measure how much nitrogen oxide loading has been applied to the storage catalyst and when the regeneration phase must be initiated, however, it is not possible to decide when the regeneration phase has to be ended. For the operation of a nitrogen oxide-storing exhaust gas cleaning system operated in rich/lean succession, the identification of the optimum point in time for ending the regeneration phase is of the greatest importance. If one ends the regeneration phase too soon, the storage catalyst is not completely emptied, and at the next storage cycle it is possible that nitrogen oxides will no longer be able to be absorbed. That means that nitrogen oxides are emitted. If one ends the regeneration phase too late, breakthrough of the reduction medium occurs, i.e., hydrocarbons are emitted, which should be absolutely avoided.
It is evident from the above, that for a complete determination of the state of a storage catalyst a sensor must deliver two signals which are independent of each other, or which can be attributed to signals independent of each other: one signal indicating the degree of loading and one signal indicating the regeneration state.
The above and other beneficial objects of the present invention are achieved by providing a method as described herein.
According to the present invention, the complex impedance Z detected by a sensor of the storage catalyst is split into two electrical quantities independent of each other.
The electrical impedance, which may be represented in a two-dimensional coordinate system, is uniquely determined by two defining quantities. Such defining quantities of the complex impedance, that are independent of each other, may include:
ohmic resistance and capacity;
ohmic resistance and amount of the impedance;
amount and real part of the impedance;
amount and phase of the impedance.
Experiments described in more detail below have shown, in particular, that, for detection of the degree of fullness (loading), the ohmic resistance is suitable as measuring quantity at a fixed frequency f0, while the signal of capacity may be used as an indicator for the end of the regeneration phase.
The two defining quantities of the electrical complex impedance, that are independent of each other, may be detected at the same and at different frequencies.
The signals which represent a measure of the nitrogen oxide loading or the regeneration state, as the case may be, are conveyed to the engine electronics. With that, a more accurate regulation may be provided than is the case with the usual model-based regulation using an NOx sensor which only indicates NOx when NOx has already had a breakthrough. In particular, one no longer has to wait until hydrocarbon substances have broken through, in order to end the regeneration phase.
As sensor, one may, for example, use the one described in German Published Patent Application No. 198 059 28 with which the impedance of a substitute material is detected, which is positioned in the exhaust gas stream, together with the storage catalyst. In view of its physical properties, the substitute material is identical or similar to the storage catalyst.
The present invention is not limited to the case of a sensor being used, which is coated with a substitute material. Rather, suitable measuring equipment may be applied on the storage catalyst, e.g., directly on the storage catalyst coating, so that the storage catalyst is diagnosed directly.
Detection of the two defining quantities of the electrical impedance, which are independent of each other, may occur after the storage catalyst or (as seen in the direction of the gas stream) in the rear area of the storage catalyst, e.g., within the last third of the storage catalyst. If storage catalyst is made of several monoliths, a sensor may be installed between two subsequent monoliths.